In the next generation of optical network infrastructures, high line rates, for example in the order of 400 Gb/s or 1 Tb/s, will be supported by so-called super-channels. Super-channels are connections that are composed of multiple optical carriers (also known as sub-carriers).
Each optical carrier may be obtained through a dedicated laser source, for example as described in a paper entitled “Software defined code-rate-adaptive Terabit/s based on time frequency packing”, by N. Sambo et al, Proceedings of the Optical Fiber Communication Conference and Exposition, and the National Fiber Optic Engineers Conference (OFC/NFOEC), March 2013. A disadvantage of generating a super-channel using multiple lasers sources is that the instability of each laser source may generate carrier frequency overlapping, which in turn may cause high signal degradation, especially in spectral-efficient transmissions such as Nyquist and Faster-than-Nyquist transmissions.
An alternative approach to using a dedicated laser for each carrier is to obtain carriers from a single multi-wavelength source, i.e. a source that is able to generate multiple carriers using a single laser source. An example of a multi-wavelength source is disclosed in a paper entitled “Flexible optical comb source for super channel systems”, by P. Anandarajah et al, OFC/NFOEC, March 2013. In contrast to the above, any instability of the laser source, exploited by the multi-wavelength source, has the advantage of not generating any carrier overlapping, given that the spacing among carriers does not change. Another advantage of using a multi-wavelength source is that the number of lasers in the network can be reduced.
In the case where carriers are generated through such a multi-wavelength source, the individual carriers must be selected from the multi-carrier signal, for example to be modulated by specific data. As an example, if 100 Gb/s information-rate carriers are assumed, each carrier should be modulated by traffic coming from a specific 100 GbE interface. Thus, a carrier has to be selected and then modulated by that specific traffic.
Moreover, in a flex-grid optical infrastructure, the spacing among carriers can be tunable (based on the rate and modulation format of each carrier). However, it is difficult to select carriers generated by a multi-wavelength source, particularly in a system where tunable channel spacing among carriers is provided, such as in a flexi-grid infrastructure.
A bandwidth variable wavelength selective switch (BV-WSS) or wavelength selective switch (WSS) can be used to select an individual carrier from a multi-wavelength source, whereby a BV-WSS or WSS is tuned to pass a particular wanted signal 100 at frequency fW, as shown in FIG. 1. However, the use of BV-WSSs and WSSs imply high costs, and they cannot be easily integrated with the source. Moreover, WSSs cannot be used in an arrangement requiring tunable carrier spacing, because WSSs work on an optical infrastructure having a fixed grid.